1. Field of The Invention
The present invention relates to the use of hexahydro-s-triazine catalysts in the preparation of flexible polyether polyurethane foams. More particularly, the instant invention relates to the use of hexamethylenetetramine as a catalyst.
2. Description of the Prior Art
Catalysts for flexible polyurethane foams generally fall into two categories: the metal salts that promote the isocyanate-polyol (gellation) reaction and the tertiary amines that promote the isocyanate-water (blowing) reaction.
Of course none of these catalysts are specific and each catalyst within a given category has catalytic activity for both competing reactions which must be balanced to achieve optimum polyurethane foam. For example, tin salts such as stannous octoate are very specific for the gellation reaction but also catalyze the reaction between isocyanates with water to form ureas and carbon dioxide. Bis(2-dimethylaminoethyl) ether is highly specific for the blowing reaction but it also catalyzes the reaction between isocyanates with polyols. Triethylenediamine (TEDA) is an example of a tertiary amine that is relatively effective in both the gellation reaction and the blowing reaction.
With high resiliency (HR) foams, where more reactive polyols are generally employed, very little tin catalysts can be used because the foam cell walls are less prone to rupture than with conventional foams, and this can result in shrinkage problems. In fact, most HR foams have to be mechanically crushed to prevent this problem. Accordingly, most or all of the tin catalyst is replaced with triethylenediamine to achieve the reaction required for final cure. Also, auxiliary tertiary amine catalysts such as pentamethyldipropylenetriamine that are significantly active for both the blowing and gellation reactions are used to decrease the overall levels of the more expensive and specific amines.
Achieving the optimum catalytic balance is particularly difficult for rapid cure, low density HR Foams which are currently of significant commercial importance.
Density reduction in such foams is achieved by either increasing the water level, and thus carbon dioxide evolution, or by the use of chlorofluorocarbons. Because of environmental concerns with the chlorofluorocarbons, the former approach is preferred. However, as the water level is increased, the exotherm increases creating a large temperature gradient between the foam core and the mold surface. Using a conventional amine catalyst like TEDA, this results in foam surface densification that extends significantly towards the core of the foam. In addition, at very high water levels, e.g. 6.5 or more parts per hundred parts of polyol, the center of the foam is discolored (scorched), presumably because of the very high exotherm and the basicity of the catalyst.
It has surprisingly been found that hexahydro-s-triazine compounds, and preferably hexamethylenetetramine (HMT), when substituted for conventional amine catalysts, greatly minimizes the densification and discoloration problems associated with low density foam employing high water levels.
Hexahydro-s-triazine compounds, and hexamethylenetetramine (HMT) in particular, are known to the art and the latter has been widely used as crosslinking agents for organic rubbers and phenolic resins. U.S. Pat. No. 4,275,169 discloses the use of HMT in the manufacture of polyester based flexible polyurethane foam with improved combustibility resistance. HMT is also disclosed in U.S. Pat. No. 3,689,440 with aromatic polyols to impart thermal stability to rigid urethane foams. The use of HMT, dispersed in waxy materials, such as stearic acid, to form large voids in polyurethane foams is disclosed in Japanese Patent No. 67020798.
The patent literature is also replete with examples of the use of hexahydro-s-triazines in polyurethane foams, particularly to induce isocyanurate formation and thus improve combustibility resistance. The most commonly cited triazine is 1,3,5-tris(3-dimethylaminopropyl)-s-hexahydrotriazine (F-DMAP) which is used to promote isocyanaurate formation. Patents that describe these uses are: U.S. Pat. Nos. 4,228,310, 4,141,862; 4,066,580, 3,981,829, and 3,723,366. None of these patents describe the use of HMT alone in polyether flexible polyurethane foams or in blends with F-DMAP.